1. Field of the Invention
This invention relates to a device for controlling internal combustion engines.
2. Description of the Related Art
In recent years, alcohol (in particular, ethyl alcohol) is drawing attention as an alternative fuel to the gasoline, from the standpoint of reducing public hazard and suppressing the use of natural resources. In many cases, alcohol is not used alone but is used as an alcohol-mixed fuel made by mixing alcohol into gasoline or the like. The alcohol ratio in the alcohol-mixed fuel is not always constant. In order to properly control an internal combustion engine, the engine control parameter for controlling the internal combustion, such as the amount of fuel injection, must be changed depending upon the alcohol ratio. Therefore, it is necessary to detect the alcohol ratio in the case that the property of the fuel has changed due to refueling.
According to Japanese Unexamined Patent Publication JP-A-2003-120363, if refueling is detected, the amount of fuel injection is temporarily increased or decreased, and then the alcohol ratio is estimated based on the behavior of the air-fuel ratio of the exhaust gas. If the alcohol ratio increases, in particular the amount of deviation in the air-fuel ratio decreases between when the amount of fuel injection is increased and when the amount of fuel injection is decreased. Therefore, in the disclosed device, a difference is detected between the air-fuel ratio of when the amount of fuel injection is increased and the air-fuel ratio of when the amount of fuel injection is decreased, and the alcohol ratio is estimated based on the difference.
Gasoline that is widely used as fuel for internal combustion engines can be grouped into a light fuel containing highly volatile light components in a large amount and a heavy fuel containing lowly volatile heavy components in a large amount. In order for the an internal combustion engine to properly operate when starting, the engine must be controlled depending upon the ratio of the light fuel and the heavy fuel. Therefore, it is necessary to detect the ratio of the light fuel and the heavy fuel in the fuel. So far, the ratio has been detected by using a heavy fuel/light fuel ratio detector that detects the ratio of the light fuel and the heavy fuel in the fuel.
When the heavy fuel/light fuel ratio detector is used, there is no guarantee that the detector will always operate properly. Therefore, a device for diagnosing the abnormal condition of the detector is necessary. At the start of the engine, the air-fuel ratio of the exhaust gas varies depending upon the ratio of the light fuel and the heavy fuel due to a difference in the volatility of the light fuel and the heavy fuel. Therefore, the above ratio can be estimated based on the air-fuel ratio of the exhaust gas, and the detector can be diagnosed if it is in abnormal condition, by comparing the estimated ratio with the ratio detected by the heavy fuel/light fuel ratio detector.
In addition to the heavy fuel and the light fuel, however, the alcohol-mixed fuel contains alcohol as the fuel. The stoichiometric air-fuel ratio of the alcohol is lower than the stoichiometric air-fuel ratio of the gasoline. Therefore, the air-fuel ratio of the exhaust gas varies depending not only upon the ratio of the light fuel and the heavy fuel in the fuel but also upon the alcohol ratio in the fuel. When the alcohol-mixed fuel is used, therefore, the above-mentioned method is not capable of correctly diagnosing if the heavy fuel/light fuel ratio detector is in abnormal condition.